The present invention relates to linear circuits; and in particular, to a linear acoustic charge transport (ACT) circuit. Generally, ACT devices have an inherent non-linear input characteristic. At high frequencies (e.g., more than a few MHz), this non-linearity causes unwanted frequency distortion that limits the dynamic range of an ACT device. FIG. 1 illustrates a typical input curve for an ACT device employing standard ohmic injection. As seen from FIG. 1, there is a non-linear relationship between the current injected into the channel of an ACT device and the voltage applied to the input of the device. These non-linearities cause second order and third order intermodulation products at the ACT device output. In many applications, these intermodulation products can cause spurious in-band signals.
There are two contributions to the input non-linearity of an ACT device. The first is termed Q-V non-linearity. This non-linearity arises from the amount of current injected into an ACT device for a given input voltage as noted above. The second contribution to the non-linear input characteristic is termed timing non-linearity. This contribution results from a variable charge injection location within the ACT channel. That is, the location of a charge packet at the instant it is created varies in dependence upon the amount of voltage applied to the input of an ACT device. Of the two contributions to the input non-linearity, the Q-V non-linearity factor dominates at lower frequencies, below 1/6 the surface acoustic wave (SAW) frequency of the ACT. The timing non-linearity dominates at higher frequencies, above 1/6 the SAW frequency of the ACT.
FIG. 2 illustrates the calculated frequency spectrum at the output of an ACT device driven by a voltage source. The data shown in FIG. 2 represents the response of an ACT device driven with a 10 MHz voltage source having a 50 .mu.A DC output and a 10 .mu.A AC output. As seen at points A and B of FIG. 2, the second and third order harmonics are quite large compared to the fundamental frequency. Thus, conventional ACT circuits have unwanted harmonics that adversely affect the system using an ACT device.